A latent heat recovery type heat exchanger mounted in a high thermal efficiency water heater is adapted to recover latent heat by providing a plurality of heat absorbing pipes within a casing which is the passage of combustion exhaust gas, arranging both pipe ends of each of the heat absorbing pipes at a predetermined vertical interval, connecting the both ends of each of the heat absorbing pipes to two headers provided on a side plate of the casing, respectively, and introducing water from external pipe to the heat absorbing pipes through the inflow header to condense moisture in the combustion exhaust gas (see, for example, PTL 1)
In such a heat exchanger, a reduction in diameter of the heat absorbing pipe is promoted in order to realize further downsizing and further improvement in thermal efficiency.
That is, the reduction in diameter of the heat absorbing pipe allows more pipes to be provided in a limited space within the casing and increases a heat transfer area of the entire heat absorbing pipe. However, the reduction in diameter of the heat absorbing pipe poses a problem in that when water is removed from the heat absorbing pipe in order to prevent water inside the heat absorbing pipe from freezing during wintertime, a water film is formed at a pipe end opening of the heat absorbing pipe due to water surface tension, causing the water to remain at a downstream portion of the heat absorbing pipe in a water flow direction. Thus, in a case where the reduction in diameter of the heat absorbing pipe is made, some measures need to be taken for smoothly removing water in the heat absorbing pipe.
FIG. 11 illustrates a conventional heat exchanger 9. The heat exchanger 9 includes a plurality of heat absorbing pipes 91 in a casing 90, each having both ends penetrating a side plate 92 of the casing 90, a downward-bending extended tubular body 93 connected to one of the pipe ends of each of the heat absorbing pipes 91 appeared from the side plate 92, and a header 94 mounted to a lower end of the extended tubular body 93 and is configured to drain water in the heat absorbing pipes 91 through the header 94 (see, for example, PTL 2). According to the conventional heat exchanger 9, even when the heat absorbing pipes 91 each having a reduced diameter are used, formation of the water film at a leading end opening of the extended tubular body 93 is prevented at drainage time by hydraulic head pressure in the extended tubular body 93, reliably removing water in the heat absorbing pipes 91.